1. Field of the Invention
This invention relates an ink composition which will exhibit excellent printability and satisfactory resistance to scumming when used in a waterless plate for lithography.
2. Prior Art
In lithography which is mainly used in the printing field, there is employed a plate consisting of hydrophilic nonimage area and oleophilic printing area. In the plate, a dampening solution is attached to the hydrophilic non-image area while an oleophilic ink is attached to the oleophilic printing area by making use of the tendency that water and an oleophilic ink are repulsive to each other and then the oleophilic ink so attached to the printing area is transferred to a to-be-printed substrate thereby obtaining a desired print.
In such conventional lithography, however, it is difficult to control the dampening solution which serves to prevent the ink from adhering to the non-image area and it constitutes a serious technical obstacle to use such a dampening solution from the viewpoint of printability and printing effects. In attempts to overcome these disadvantages, there have heretofore been proposed various lithographical methods using waterless lithographical plates. As the newest and practicable lithographical method using a waterless plate, there has been proposed a printing method using a plate in which the non-image area are covered with non-tacky thin films of silicone rubber or the like. In this printing method so proposed, it is reported that printing can be effected with a usual oleophilic ink without the supply of a dampening solution It has now been found undesirable, however, that scumming will disadvantageously be caused when a conventional oleophilic ink is used for printing since the printability of the conventional ink is unsuitable.
The reason why scumming is caused in waterless lithography is that the ink used lowers in cohesion due to a rise of the plate in temperature derived from the friction of the moving parts and rolls of the printing press used during printing whereby the ink adheres to the non-image area which are originally repulsive to such inks.
To avoid this drawback, there have been proposed a method using a printing ink incorporated with a silicone compound (Japanese Pat. Gazette No. 50-11287, Japanese Pat. Appln. Laid Open Gazette No. 55-31885), a method using a printing ink which contains a resin modified with a silicone compound (Japanese Pat. Gazettes Nos. 51-10124, 52-10041 and 52-10042), and the like. These methods, however, have still not satisfactorily solved problems as to scumming and have left unsolved problems as to unsatisfactory levelling of inks, unsatisfactory gloss, and the like. There has also proposed an improved method in which is used a varnish having gelled by incorporation with, for example, and aluminum compound such as aluminum stearate or aluminum octoate. When high anti-scumming property is attempted to be attained with the above improved method, the ink used will greatly increase in viscosity whereby are raised problems as to ink transferability such as transfer of the ink on the printing press or to the plate or blanket, this proving that said improved method is still not satisfactory at the present.